Electric power steering devices (EPS) are required to have a motor that rotates smoothly and quietly. In many cases, brushless motors are used as the drive source for electric power steering devices. Typically, such a motor is controlled via a sine wave energization applied to each phase (U, V, W) based on the current control in the d/q coordinate system.
When there is an upper limit for the output voltage of the motor control device, the required output voltage can exceed the highest value (for example, power voltage) of voltage applicable to a drive circuit, for example, at high speed rotation of the motor. This phenomenon is referred to as voltage saturation. Voltage saturation can cause torque ripple and unpleasant noise.
To suppress the occurrence of voltage saturation, a voltage limiting process for limiting a voltage command value (guard against voltage saturation) is conventionally executed. For example, a limit value in the voltage limiting process is determined in advance based on the standards and specifications of a drive circuit. As shown in FIG. 9, voltage saturation occurs when the resultant vector Vdq* of a d-axis voltage command value Vd* and a q-axis voltage command value Vq* exceeds a voltage limit value Vdq_lim, which is set in advance. At this time, the d-axis voltage command value Vd* and the q-axis voltage command value Vq* are each corrected such that the resultant vector Vdq* after the correction is less than or equal to the voltage limit value Vdq_lim.
FIG. 9 shows an example in which the q-axis voltage command value Vq* is lowered while maintaining the d-axis voltage command value Vd*. Correction may be executed in other manners. For example, the d-axis voltage command value Vd* and the q-axis voltage command value Vq* may be corrected to maintain the angle of the resultant vector Vdq* after voltage saturation starts.
Problems caused by such voltage saturation can be avoided by expanding the rotation range of the motor through execution of the field weakening control. For example, Patent Document 2 discloses a motor control device (steering device) that initiates the field weakening control based on a q-axis current deviation. The motor control device disclosed in Patent Document 3 computes a voltage saturation factor, which indicates the degree of voltage saturation, based on a voltage command value used in current control. The phase of the current command value is corrected in accordance with the voltage saturation factor, so as to lower the current command value as well as to weaken the field. As a result, voltage saturation is effectively prevented.